Why extreme water levels are rapidly rising on the Estonian coasts of the Baltic Sea?

A doctoral thesis to be defended at TTÜ explains the reasons why extreme water levels are rapidly rising on the Estonian coasts of the Baltic Sea.

A PhD student of TTÜ Department of Cybernetics Katri Pindsoo will defend her doctoral thesis “Spatio-Temporal Changes in the Components of Extreme Water Levels on Estonian Coasts” on 6 October.A specific feature of the Baltic Sea is that certain sequences of storms may push water into this sea via the Danish straits in such an amount that the water level in the entire sea will rise by up to 1 m. On top of the resulting severely elevated water levels and typical storm surges, wave-induced processes contribute to the total water level in some coastal sections. The urbanised areas on gently sloping shores open to waves, which include also Pärnu, Haapsalu and several new districts in Tallinn, are most exposed to risks from extreme water levels.

The thesis makes an attempt to establish the role of the major components or combinations thereof contributing to the water level extremes and associated risks on the Estonian coasts.

The supervisor of the doctoral thesis, Head of Laboratory of Wave Engineering and Professor of Department of Civil Engineering and Architecture, Academician Tarmo Soomere says, “We can sleep soundly until these three components do not act all together. It appears from the thesis that in the coastal areas open to the east one of these components is always absent, in other words, the increase rate of extreme water levels cannot be very high. However, in the coastal areas open to south-west, west or north the components may occur simultaneously. Wave set-up is the most insidious phenomenon. It may pose a risk to about 50% of coastal segments and may additionally raise the already high water level by 70-80 cm in some coastal segments of the city of Tallinn. This kind of water level rise is highly sensitive with respect to orientation of the coastline, making it very difficult to measure.”

The thesis examines how extreme wave set-up events have changed along the shores of Tallinn Bay and Muuga Bay since 1980. The highest set-up events generated until the year 2012 in the coastal sections open to the east occurred in the 1980s. Almost all these maxima were overridden in the years 2012–2014, which means that strong eastern storms have reappeared in the Gulf of Finland after a 30-year period.

The water level in oceans rises approximately 1–3 mm/yr. The annual maximum water levels on the coast of Estonia increase much more rapidly: at an average rate of 4–10 mm/yr depending on the location. The contribution of the water volume pushed into the Baltic Sea by storms to this increase is about 4 mm/yr. The contribution of storm surges into this increase varies largely from almost zero on the shores of the Western Estonian archipelago up to 6 mm/yr in the eastern part of the Gulf of Finland and the Gulf of Riga.

“This unusual and somewhat unexpected pattern can be attributed to an increase in the duration of series of storms that force the North Sea water into the Baltic Sea and the rotation of wind directions causing some of high storm surges. For example, in the Gulf of Finland the likely “culprits” are the few very strong western storms. However, this does not mean that the previously occurring pattern of strong winds has disappeared, simply other kind of storms and their series have appeared in the system,” Professor Soomere explains.

The supervisor of the doctoral thesis is Academician Professor Tarmo Soomere (TTÜ).